1. Field of the Invention
The present disclosure is related to systems for physically characterizing a sample by a probe which is brought in contact with or in close proximity with the surface of the sample. These characterization systems can be a scanning probe system such as a scanning probe microscope, a profilometer or an atomic force microscope. The present invention is in particular related to a probe used in such characterization systems and a method of manufacturing such a probe.
2. Description of the Related Technology
Scanning proximity microscopes or scanning probe microscopes such as an atomic force microscopy (AFM), a scanning tunneling microscope (STM), a magnetic force microscope (MFM), a spreading resistance microscopy (SSRM) probe, operate by scanning the surface of a sample with a probe having a small tip. The probe configuration typically comprises a mounting or holding block to which a cantilever, also known as stylus, is mounted. Attached to this cantilever is a tip, which is pointing towards the sample surface when scanning this surface. This tip preferably has a high hardness and low wear out. The tip and the holding block are mounted to opposite ends along the length of the cantilever. During the scanning of the surface, the sample is moving relative to the tip either by movement of the sample only, by movement of the tip or by a combined movement of both tip and sample.
Such a probe can be used for measuring the topography of the sample's surface by sliding the probe over the surface and monitoring the position of the tip at each point along the scan line. In this application the conductive properties of the tip are less relevant and dielectric or semiconductor materials can be used to manufacture the tip. The probe can also be used for determining the electrical properties of a sample, e.g. the resistance and electrical carrier profile of a semiconductor sample or for nano-potentiometry measurements of the electrical distribution of sample, e.g. a semiconductor sample. For these applications at least the tip of the probe is conductive.
Boron-doped diamond tips are required for two-dimensional carrier profiling of semiconducting devices using scanning spreading resistance microscopy (SSRM) as they provide a highly conductive tip with high hardness and low wear out. The diamond probe therefore remains an essential component for SSRM. Over the past years the SSRM method has been further developed to allow measurements in vacuum, measurements of silicon and III-V semiconducting materials, and measurements of nanowire structures.
A fabrication process for metal cantilevers with integrated diamond pyramidal tips is known. Although an electrical resolution of 1 nm has been demonstrated, this overall process (FIG. 5) suffers from low yield, high manufacturing complexity and high manufacturing cost because a very sharp pyramidal tip is needed. This pyramidal tip is made by a molding technique whereby first a pyramidal etch pit (15) is formed in a supporting silicon substrate (14) by anisotropic etching through an opening in an hardmask (3) (FIGS. 5a-d). This etch pit is then used as a mold and is filled with diamond (16) to create the pyramidal diamond tip (FIG. 5e). The diamond layer is then patterned (17) to form the tip (7) portion of the probe configuration (FIG. 5f). A cantilever (8) is attached to the diamond tip. Thereafter the silicon mould is removed thereby releasing the pyramidal diamond tip and the cantilever attached to it.
This approach for manufacturing diamond probe tips requires a perfectly square patterning of a micrometer sized area (FIG. 1) which requires a lithography stepper and an expensive mask. The required symmetry of the square pattern of a few nanometers can actually not be specified by the mask maker, as the square is 7 microns×7 microns and to pattern that this with nanometer precision is not possible. The use of an etch mask such as silicon oxide layer exposing the underlying silicon substrate, can also cause a non-symmetrical etch pit (1) and as a result a knife-shaped tip (2) (FIGS. 2a-b). Furthermore, particle-like residues (5) are left behind by the anisotropic KOH etching which can remain at the bottom of the tip, i.e. at the position of the apex (2) (FIG. 3). The etch pit is also more difficult to fill perfectly at the bottom with diamond. All these factors together contribute to the low overall yield. Furthermore, the tip is not visible during scanning in SSRM measurements which reduces the tip lifetime as a lot of time is being spent when bringing the tip to the desired area to be measured (FIG. 4). FIG. 4 shows a schematic top (4a) and side view (4b) of a prior art probe configuration (6) comprising a holder block (9) mounted to one main surface (12) of the cantilever (8), while a tip (7) is attached to the opposite main surface (11) of the cantilever (8). The holder block (9) and the tip (7)
Furthermore, the tip is not visible during scanning in SSRM measurements which reduces the tip lifetime as a lot of time is being spent when bringing the tip to the desired area to be measured (FIG. 4). FIG. 4 shows a schematic top (4a) and side view (4b) of a prior art probe configuration (6) comprising a holder block (9) mounted to one main surface (12) of the cantilever (8), while a tip (7) is attached to the opposite main surface (11) of the cantilever (8). The holder block (9) and the tip (7) are located at opposite ends along the longitudinal direction x of the cantilever (8). The tip (7) protrudes in a direction z, perpendicular to the x-y plane of the cantilever the tip as can be seen in FIG. 4b. 
Another important problem is the need for a micro-fabricated probe for nanoprober characterization systems. A nanoprober typically comprises a scanning electron microscope for viewing the surface of the sample (10) to be probed or scanned, nanomanipulators comprising the probe configuration for contacting the surface (18) and parameter analyzer(s) for performing electrical measurements of the sample via the nanomanipulators. The tip (22), in particular the apex (2) thereof, is hidden underneath the cantilever and hence the scanned area of the sample (10) is not visible during scanning. Therefore, a lot of time is spent on searching for the region of interest using only the scanned images as references. This leads to early tip wearing and strongly reduces tip lifetime. So far, only manually etched tungsten probes (22) are available as probe tips for the nanoprober (FIG. 6). The tip sharpness is limited to about 20-100 nm. There is a strong need for microfabricated tips instead of manually fabricated tips. Moreover alternative materials besides tungsten should be usable as tungsten tips suffers from oxidation. For measurements on silicon for example, conductive diamond probes are needed. The tips should also be sharper to improve resolution of the measurement.